S
93102
931020
For Supervisor’s use only
Scholarship 2009
Chemistry
9.30 am Saturday 28 November 2009
Time allowed: Three hours
Total marks: 48
Check that the National Student Number (NSN) on your admission slip is the same as the number at the
top of this page.
You should answer ALL the questions in this booklet.
A periodic table is provided on page 2 of this booklet.
Write all your answers in this booklet.
If you need more space for any answer, use the page(s) provided at the back of this booklet and clearly
number the question.
Check that this booklet has pages 2–25 in the correct order and that none of these pages is blank.
You are advised to spend approximately 30 minutes on each question.
YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION.
© New Zealand Qualifications Authority, 2009
All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
Be
9.0
K
Rb
Cs
Scholarship Chemistry 93102, 2009
Fr
Ca
Sr
Ba
Ra
Actinide
Series
Y
Lu
La
Ac
Lr
Zr
Hf
Ce
Th
Rf
Nb
Ta
Pr
Pa
Db
W
Nd
U
Sg
Mn
7
Tc
Re
237
Np
Bh
Ru
Os
239
Pu
Hs
Rh
Ir
Eu
Mt
241
Pd
Pt
244
Cm
Bk
Au
Cf
Hg
Ga
Tl
Ge
Pb
257
Fm
P
Sb
Bi
Tm
258
Md
S
259
No
Po
210
84
Yb
Te
128
52
79.0
Se
32.1
34
102
O
16
16.0
16
8
173
70
209
83
122
51
74.9
As
31.0
33
101
N
15
14.0
15
7
169
69
207
82
119
Sn
72.6
50
100
Si
28.1
32
Er
C
14
12.0
14
6
167
68
204
81
Ho
In
115
49
69.7
31
Es
Al
27.0
252
99
B
13
10.8
13
5
165
67
201
80
Dy
Cd
112
48
251
98
Zn
12
65.4
30
163
66
197
79
Tb
Ag
108
47
249
97
Cu
11
63.5
29
159
65
195
78
106
46
Gd
96
Ni
10
58.7
28
157
64
268
109
192
77
103
45
Am
95
Co
9
58.9
27
152
63
265
108
190
76
101
44
Sm
94
Fe
55.9
26
8
Molar Mass / g mol–1
150
62
264
107
186
75
98.9
43
54.9
Pm
93
H
1.0
25
1
147
61
263
106
184
74
95.9
Mo
42
238
92
Cr
6
52.0
24
144
60
262
105
181
73
92.9
41
231
91
V
5
50.9
23
141
59
261
104
179
72
91.2
40
232
90
Ti
4
47.9
22
140
58
262
103
175
71
88.9
39
227
89
Sc
3
45.0
21
139
57
226
88
137
56
87.6
38
40.1
20
Lanthanide
Series
223
87
133
55
85.5
37
39.1
19
Mg
24.3
12
4
Na
6.9
Li
2
23.0
11
3
1
Atomic Number
PERIODIC TABLE OF THE ELEMENTS
F
Br
I
At
210
85
127
53
79.9
35
35.5
Cl
19.0
17
9
17
He
Ar
Kr
Xe
Rn
222
86
131
54
83.8
36
40.0
18
20.2
Ne
4.0
10
2
18
2
3
You have three hours to complete this examination.
QUESTION ONE (8 marks)
(a)
(i)
(ii) In VSEPR theory it is assumed that the bond from the central atom to an atom of lower
electronegativity occupies more space than the bond from the central atom to an atom of
higher electronegativity.
Draw a 3-dimensional structure for each of the possible isomers of the pentahalide
PCl3F2, indicating the size of the F-P-F bond angle in each isomer.
Comment on the polarity of each of the isomers of PCl3F2 and identify the isomer that is
most likely to occur.
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(iii) Upon standing for several days at low temperature, phosphorus pentahalide compounds
convert to isomeric ionic solids.
In crystalline PBr4Cl, only one of the two ions formed contains phosphorus.
Predict the formulae of the ions in solid PBr4Cl, and justify your answer.
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(b) Two possible isomers may be formed when H+ reacts with isocyanate, NCO–, as the H+ may
bond to either nitrogen or oxygen. A structural study of one of the isomers that is produced
shows that it has a bond angle of 105° at the atom to which the H is attached.
•
Draw Lewis diagrams for the two possible structures of the product.
•
Identify and explain which Lewis diagram better represents the structure of the product
with the bond angle of 105° mentioned above.
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QUESTION TWO (8 marks)
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(a) Titanium is a Group 4 transition metal. There are three different chlorides of titanium: TiCl2,
TiCl3 and TiCl4. One of these chlorides, A, is a solid that dissolves in water to produce a
mildly acidic purple solution. On standing in the presence of air, the colour of this solution
fades, and a white solid, TiO2, is formed. The chlorides B and C are very reactive toward
water. B is a liquid and reacts to produce a strongly acidic solution and TiO2. C reacts with
acidified water to produce a purple solution and hydrogen gas.
(i) Identify which of the chlorides of titanium is A, which is B and which is C.
Justify your answers using the properties of transition metals, including the colours
and reactions outlined above, and / or by analogy with the chemistry of other transition
metals. Include balanced equations for the reactions described.
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(ii) Determine the upper limit for the reduction potential for each of the Ti(IV) / Ti(III)
couple and the Ti(III) / Ti(II) couple.
Eo(H2O / H2) = –0.42 V
Eo(O2 / H2O) = 0.82 V
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(b) Titanium occurs naturally both as ilmenite (FeTiO3) and TiO2. The isolation of titanium
metal from ilmenite involves heating ilmenite, chlorine and carbon to form TiCl4. This is then
converted into titanium metal by treatment with magnesium.
2FeTiO3 + 7Cl2 + 6C → 2TiCl4 + 2FeCl3 + 6CO
Discuss the possible oxidation number changes and electron transfers that occur in the
conversion of ilmenite to TiCl4, taking into consideration the variety of oxidation numbers
that are possible for the metals.
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QUESTION THREE (8 marks)
(a)
(i)
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The structural formula of the amino acid glycine can sometimes be written as
(a zwitterion) and sometimes as H2NCH2COOH.
Explain which structure is more appropriate, taking into account functional group
chemistry and the fact that glycine is a crystalline solid that has a melting point of
233°C.
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(ii) The amino acids serine and asparagine have the zwitterion structures shown below.
+
H3N CH COO–
+
H3N CH COO–
CH2
CH2
C O
OH
NH2
serine
asparagine
These amino acids can be linked to form two different dipeptides.
Discuss how the structures of these dipeptides change as the pH of the aqueous
solutions change from highly acidic, through neutral, to highly basic.
Include structural formulae in your answer.
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(b) Esters are hydrolysed by reaction with dilute acid. This reaction and the purification of the
organic products involve the use of both reflux and distillation.
Discuss how each of these two processes is involved in preparing and isolating pure samples
of the organic products.
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(c)
Compound A has the formula C9H18O3.
When compound A is refluxed with dilute sulfuric acid, it forms compound B, C6H12O3 and
compound C, C3H8O. Both compounds B and C react with acidified potassium dichromate
to produce compounds D, C6H10O3 and E, C3H6O respectively. Neither compound D nor E
reacts with Tollens’ reagent.
When compound B is reacted with concentrated sulfuric acid, it produces THREE structural
isomers F, G and H, of molecular formula C6H10O2, all of which are optically active. F exists
as geometrical isomers, but G does not. Both F and G decolourise a solution of bromine, H
does not.
Give the structural formulae for compounds A to H that are consistent with the information
above.
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Space for working.
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QUESTION FOUR (8 marks)
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(a)
Models are representations used to explain observed phenomena.
A model useful for describing the physical properties of Group 18 elements views the
particles of these elements as individual atoms.
In contrast, various properties of metals, both in solid and liquid form, can be explained by
a model that views the structure of the metal as cations submerged in a “sea of electrons”.
In this model, the “electron sea” consists of valence electrons moving freely throughout the
metal structure.
The table below shows the melting points (mp) and boiling points (bp) for selected elements
in Groups 1 and 18 of the periodic table.
1
2
13
14
15
16
17
18
Li
mp: 180°C
bp: 1342°C
Be
Ne
mp: –249°C
bp: –246°C
Na
mp: 98°C
bp: 883°C
Mg
Ar
mp: –189°C
bp: –186°C
K
mp: 63°C
bp: 760°C
Ca
Kr
mp: –157°C
bp: –152°C
Rb
mp: 39°C
bp: 686°C
Sr
Xe
mp: –112°C
bp: –108°C
(i)
Explain the trend in boiling points of the Group 18 elements.
(ii) Discuss how each of the statements below is evidence for the different models described
above.
•
The boiling point of a Group 18 element is significantly lower than the boiling
point of the Group 1 element with the next higher atomic number.
•
The difference between the boiling point and the melting point of a Group 1
metal, such as sodium, is much larger than the difference between the boiling
point and melting point of a Group 18 element, such as argon.
(iii) Predict, using the “electron sea” model described above, how the boiling points for the
Group 1 metals would compare with those for the Group 2 metals.
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(b) The standard enthalpy of atomisation, ΔatH , is defined as the enthalpy change when 1 mole
of atoms, in the gas phase, is formed from the element in its standard state at 25°C.
Element
Standard Enthalpy of
Atomisation, ΔatH / kJ mol–1
Bond Enthalpy
/ kJ mol–1
C
717
C–F
485
F
79
C–Cl
328
Cl
122
Calculate the standard enthalpy of formation, ΔfH , of trichlorofluoromethane, CCl3F(g).
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QUESTION FIVE (8 marks)
(a)
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When silver ions are dissolved in an aqueous ammonia solution, complex ions of
Ag(NH3)2+(aq) form. The formation of Ag(NH3)2+(aq) occurs in two steps that are
represented by the equations below, together with the corresponding equilibrium constant
for each reaction.
Ag+(aq) + NH3(aq)  Ag(NH3)+(aq)
K1 = 2.1 × 103
Ag(NH3)+(aq) + NH3(aq)  Ag(NH3)2+(aq)
K2 = 8.2 × 103
0.15 mol of AgNO3(s) is dissolved in 1.00 L of a 1.00 mol L–1 solution of aqueous ammonia.
Use the values of the equilibrium constants to identify the major species in this solution at
equilibrium, and hence calculate the concentrations in mol L–1 of the Ag+, Ag(NH3)+ and
Ag(NH3)2+ ions.
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(b) 15.35 g of a mixture of sodium nitrate, NaNO3, and magnesium nitrate, Mg(NO3)2, was
heated until no more gases were evolved. The NaNO3 decomposes giving sodium nitrite,
NaNO2, and oxygen gas, while the Mg(NO3)2 decomposes to give the metal oxide, nitrogen
dioxide and oxygen. The water-soluble part of the residue produced on heating was used
to prepare 1.00 L of solution. 10.00 mL of this solution was reacted with 20.00 mL of
0.0200 mol L–1 acidified potassium permanganate (which oxidises nitrite to nitrate). The
excess potassium permanganate required 10.25 mL of 0.0500 mol L–1 oxalic acid, H2C2O4,
for complete reaction in which oxalic acid is oxidised to form CO2.
Write balanced equations for all of the reactions occurring above, and hence calculate the
mass, in grams, of each metal nitrate present in the original mixture.
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QUESTION SIX (8 marks)
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Three flasks contain aqueous solutions of the same pH. One of the solutions is 0.0010 mol L–1
nitric acid, one is 0.0060 mol L–1 methanoic acid (HCOOH) and one is 0.040 mol L–1 anilinium
hydrochloride (C6H5NH3Cl).
(a)
Use the information above to calculate the pKa for HCOOH and for C6H5NH3+.
Explain why the different concentrations of the three acids produce the same pH.
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(b) The three acid solutions in part (a) are diluted by a factor of 10.
Discuss the change in both the pH of each of the solutions and the concentrations of the
species present.
NO CALCULATIONS ARE REQUIRED.
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Clearly number the question.
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number
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number
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number
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number
Scholarship Chemistry 93102, 2009
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93102
For Assessor’s Use Only
Question Number
Marks
ONE
(8)
TWO
(8)
THREE
(8)
FOUR
(8)
FIVE
(8)
SIX
(8)
TOTAL
(48)
Scholarship Chemistry 93102, 2009
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